Senescence is the final stage that plants undergo during their lifetime. The initiation of senescence can be said to be a rapid changeover point in a plant's development stage. As senescence progresses, a plant's synthesis ability gradually decreases and it loses cellular homeostasis with successive degradation of intracellular structures and macromolecules, finally leading to death (Matile P. et al., Elservier, 413–440, 1992; Nooden L. D. et al., Academic press, 1988; Thiman K. V. et al., CRC press, 85–115, 1980; and Thomas H. et al., Annu. Rev. Plant Physiol., 123:193–219, 1993). Such senescence of plants is a series of continuous biochemical and physiological phenomena, which is genetically destined to progress in highly intricate and active manners at cell, tissue and organ levels. However, the senescence of plants is seen as a process of cellular degeneration, and at the same time, a genetic character which is actively acquired for adaptation to environment during the development process, including migration of nutrients from growth organs to genital organs at the winter season.
The suppression of plant senescence is not only of great scientific importance in itself, but also of great industrial importance in terms of the productivity of crops or the possibility of improving post-harvest storage efficiency. For this reason, genetic, molecular biological, physiological and biochemical studies have been actively conducted in the attempt to establish plant senescence phenomena. However, reports regarding phytohormones are the main area of interest; studies on senescence regulation, such as the induction of senescence regulation using senescence regulatory genes, are, as yet, insufficient.
Cytokinin, a plant growth hormone, is known as a hormone capable of physiologically delaying senescence. For this reason, there have been studies conducted to delay senescence by regulation of cytokinin synthesis, but there were problems in that other physiological actions are also affected due to the influence of hormones. However, there has been recent success in delaying the progression of senescence by a method in which an IPT gene is linked to a promoter of a senescence-specific SAG12 gene so that the synthesis of cytokinin is specifically regulated at a certain senescence stage. In the case of tobacco plants whose senescence was delayed by this method, an increase of more than 50% in productivity could be achieved while causing little or no changes in the blooming time and no other deformations (Gan S. et al., Science, 22:1986–1988, 1995). Moreover, plants for delay of senescence have been developed, making the ripe tomatoes a main object of this development. For such development, the following methods have been applied; inhibition of synthesis of ethylene, a phytohormone playing an important role in senescence, or reduction of the amount of intracellular ethylene (Klee et al., Plant Cell, 3(11): 1187–93, 1991; and Picton et al., Plant Physiol., 103(4): 1471–1472, 1993).
In addition, studies on delay of senescence are mainly focused on the manipulation of degradation-associated genes, which have activities associated with biochemical changes occurring in a process of senescence or are involved in the signal transduction system. A typical example connected with such studies includes commercialized tomatoes, called “Flavr savr”, in which the expression of polygalacturonase gene involved in the degradation of cell walls is impeded using antisense DNA so that the softening of tomatoes is prevented, thereby improving the transport and storage properties of tomatoes (Giovannoni et al., Plant Cell, 1(1): 53–63, 1989). It was also reported that, where the expression of phospholipase D involved in degradation of lipids is impeded with the antisense DNA, senescence caused by phytohormones is delayed (Fan et al., Plant Cell, 9(12): 2183–96, 1997). Furthermore, it was recently reported that leaf senescence is delayed, in tobacco plants in which SAG12 promoter, and kn1 (knotted 1), a homeobox gene of corn, are expressed (Ori et al., Plant Cell, 11:917–927, 1999).
However, a method capable of more directly regulating senescence involves isolating mutant of senescence-associated genes and analyzing genes which cause the mutation. According to existing reports, it is known that, in Arabidopsis thaliana, the expression of ethylene receptors is controlled in a ripening period of fruits or in the senescence process of flowers (Payton S. et al., Plant Mol. Biol., 31(6): 1227–1231, 1996), and the expression of clp gene is regulated in a senescence process of leaves. Recently reported were studies on the identification of genes involved in a senescence process of leaves (Oh S. A. et al., Plant Mol. Biol., 30(4): 739–754, 1996), and on the isolation of leaf senescence-delaying mutants from Arabidopsis thaliana (Oh S. A. et al., The Plant Journal, 12(3):527–535, 1997). Also, a mutant gene was successfully isolated from an ore9 mutant of the leaf senescence-delaying mutants (Woo H. R. et al., Plant Cell, 13: 1779–1790, 2001). In addition, activities of a promoter of sen1, a senescence-associated gene, were reported (Oh S. A., et al., Journal of Plant Physiology 151:339–345, 1997). However, studies on genes that directly regulate senescence and their functions are, as yet, insufficient.